A compressor is a machine which accelerates the particles of a compressible fluid, e.g., a gas, through the use of mechanical energy to, ultimately, increase the pressure of that compressible fluid. Compressors are used in a number of different applications, including operating as an initial stage of a gas turbine engine. Among the various types of compressors are the so-called centrifugal compressors, in which the mechanical energy operates on gas input to the compressor by way of centrifugal acceleration which accelerates the gas particles, e.g., by rotating a centrifugal impeller through which the gas is passing. More generally, centrifugal compressors can be said to be part of a class of machinery known as “turbo machines” or “turbo rotating machines”.
Centrifugal compressors can be fitted with a single impeller, i.e., a single stage configuration, or with a plurality of impellers in series, in which case they are frequently referred to as multistage compressors. Each of the stages of a centrifugal compressor typically includes an inlet conduit for gas to be accelerated, an impeller which is capable of providing kinetic energy to the input gas and a diffuser which converts the kinetic energy of the gas leaving the impeller into pressure energy.
FIG. 1 schematically illustrates a multistage, centrifugal compressor 10. Therein, the compressor 10 includes a box or housing (stator) 12 within which is mounted a rotating compressor shaft 14 that is provided with a plurality of centrifugal impellers 16. The rotor assembly 18 includes the shaft 14 and impellers 16 and is supported radially and axially through bearings 20 which are disposed on either side of the rotor assembly 18.
The multistage centrifugal compressor 10 operates to take an input process gas from duct inlet 22, to accelerate the particles of the process gas through operation of the rotor assembly 18, and to subsequently deliver the process gas through outlet duct 24 at an output pressure which is higher than its input pressure. Between the impellers 16 and the bearings 20, sealing systems 26 are provided to prevent the process gas from flowing to the bearings 20. The housing 12 is configured so as to cover both the bearings 20 and the sealing systems 26 to prevent the escape of gas from the centrifugal compressor 10. Also seen in FIG. 1 is a balance drum 27 which compensates for axial thrust generated by the impellers 16, the balance drum's labyrinth seal 28 and a balance line 29 which maintains the pressure on the outboard side of the balance drum 27 at the same level as the pressure at which the process gas enters via duct 22.
Various types of process gasses may be used in the multistage centrifugal compresses. For example, the process gas maybe any one of carbon dioxide, hydrogen sulfide, butane, methane, ethane, propane, liquefied natural gas, or a combination thereof. When operating with a corrosive process gas, centrifugal compressors can employ impellers which are composed of corrosion resistant alloys, e.g., stainless steels, nickel based super alloys and titanium alloys. However, the materials used in these corrosion resistant alloys tend to be expensive.
Attempts at alternative solutions have also included the use of coatings to improve corrosion resistance and attaching a cladding layer to counteract stress corrosion cracking. However, these methods have not been shown to be effective on the flow path parts of an impeller due to the complexity of the geometry, which can result in partial or no coverage, and because of the deformation caused to the impeller when applying the cladding.
Accordingly, systems and methods for reducing costs while maintaining acceptable material properties for such working environments are desirable.